The /clang: suffix passed to Clang-CL was accidentally also passed to
the regular standalone Clang, which caused compilation to fail. We now
pass /W3 to Clang-CL, which it interprets as -Wall.
The _CRT_SECURE_NO_WARNINGS macro is now defined for both Clang and
Clang-CL.
The /entry: flag passed to link.exe is now also passed to lld-link,
letting the windows subsystem tests and examples link.
Fixes#1807.
Closes#1824.
Closes#1874.
(cherry picked from commit 061a0263a9)